Hydride Theory
Hydrides are chemical compounds formed when hydrogen gas reacts with certain other substances. Usually the substance is a metal such as Ca, Mg, Li, U, V, LaNi.sub.5, FeTi etc. Other organic and inorganic chemicals are known to form hydrides. The present invention can make use of any of the hydride forming substances however metal, specifically LaNi.sub.5 is used for illustrative purposes.
The phase changes during formation and decomposition of the metal hydrides is accompanied by significant flows of heat. This heat of reaction must be rejected (formation) or provided (decomposition) by an external heat sink or source for the phase change to occur. For example, approximately 210 Joules (watt-seconds equivalent to 0.1991 BTU) of heat is generated by the exothermic reaction from converting one gram of Lanthium Nickel (LaNi.sub.5) into a hydride (LaNi.sub.5 H.sub.6) and must be rejected to an external heat sink. Conversely, approximately the same amount of heat must be absorbed from an external source by the hydride in order to release the hydrogen and return to the metal state.
The absorption and desorption of hydrogen can be controlled by variations in hydrogen pressure or temperature of the metal. The relationship between pressure and temperature at equilibrium is given approximately in accordance with the van't Hoff equation: EQU In(P)=A-B/T
where P is the pressure of the hydrogen in kilo-Pascal (kPa), T is temperature in Kelvin, A and B are constants depending upon the hydride. Variation of either the pressure or temperature causes a corresponding change in the other parameter in accordance with this equation, resulting in a new equilibrium condition, providing that a means is available to accommodate the heat generated or liberated that accompanies the change.
Each type of hydride will typically have a unique set of technical information (e.g. equilibrium disassociation pressure characteristic, (EDP) available from the supplier (for example, Ergenics Inc., Ringwood N.J.) that will allow solution of the van't Hoff equation for equilibrium pressure at the design temperatures. The technical data will also include the heat requirements (reaction energy) of the hydride in the form of Kilo-Joule per gram (kJ/g) or BTU/lb of hydride. This information is used in the thermal design of the system to determine the quantity of hydride required to make the desired heat exchange.
Heat Pump
A hydride heat pump is based upon the concept of transferring hydrogen gas between two different, thermally isolated hydrides. The hydrogen is forced out of one hydride (hydride A.sub.h in the discussion that follows) in a manner that allows it to be absorbed by a second hydride (hydride B.sub.h in the following). It is then forced out of the second hydride at which time it is reabsorbed into the first hydride. The forcing action is provided by external heat or pressure sources. Typically, heat is provided to force the gas out of the first hydride and heat must be absorbed from a source (refrigeration) to enable the release from the second hydride.
Heat pump requirements are stated in terms of amount of heat that must be moved in a given period of time-for example BTU/hr. Large amounts of heat will result in requirements for large amounts of hydride or for the use of smaller amounts that are cycled frequently enough over a period of time to pump the equivalent amount of heat. For example one gram of hydride (LaNi.sub.5 H.sub.6) can be used to pump approximately 0.2 BTU per cycle. If the pump is able to cycle 20 times an hour, 4 BTU per hour can be pumped with one gram instead of twenty grams cycling once per hour. Cost and complexity of hydride systems have limited their use in the past. Both cost and complexity are reduced with a design prioritized to allow for rapid cycling.
Shown diagrammatically in FIG. 1 are the typical van't Hoff temperature and pressure profiles for typical hydrides, A.sub.h and B.sub.h. The hydrides are chosen such that the equilibrium pressure of one (hydride B.sub.h) is appreciably higher than the other (hydride A.sub.h) at the same temperature at which each will be rejecting heat to the environment (T.sub.s).
The heat pump is designed to operate between three temperatures. The first temperature T.sub.s, is the sink temperature, usually that of the external environment to which heat will be rejected from each hydride in sequence. The second is the temperature T.sub.h to which one of the hydrides (hydride A.sub.h) must be raised, providing the heat necessary to cause it to desorb all of its hydrogen at its equilibrium pressure. The third, T.sub.c, is the temperature at which the desorbing hydride (hydride B.sub.h) will extract heat from its environment (heat exchanger).
At one step of a complete cycle.sub.1 hydride A.sub.h is thermally connected to a source of high temperature (T.sub.h). T.sub.h is the temperature which causes hydride A.sub.h to desorb hydrogen at its equilibrium pressure P.sub.h. With hydrogen pressure at P.sub.h connected to a chamber containing hydride metal B.sub.h, hydride B.sub.h will absorb gas (forming the hydride phase) providing it can reject the heat of formation to the heat sink at T.sub.s. Hydride B.sub.h, is therefore simultaneously thermally connected to the external environment, at temperature T.sub.s, rejecting the liberated heat.
When hydride B.sub.h is fully charged (has absorbed all the hydrogen it is capable of at temperature T.sub.s,, it is disconnected from the heat sink and thermally connected to T.sub.c, the environment to be cooled. Hydride A.sub.h is simultaneously disconnected from the T.sub.h source and connected to the environment at temperature T.sub.s, while simultaneously in hydrogen communication with hydride B.sub.h.
Hydride A.sub.h.spsb.1, at temp. T.sub.h,, will reject heat to the environment at T.sub.s, and will attempt to reach equilibrium at pressure P.sub.l and begin to absorb hydrogen. As the gas pressure drops to P.sub.1, desorption of hydrogen from hydride B.sub.h draws heat from its environment as it attempts to reach its equilibrium temperature T.sub.c and providing hydrogen for hydride A.sub.h to absorb. The cycle repeats when hydride B.sub.h is discharged (hydrogen desorbed) and hydride A.sub.h is fully charged again.
Hydrides A.sub.h and B.sub.h typically communicate thermally with the heat sources and sinks by means of piping and valves controlling the flow of a thermal exchange media (working fluid or fluids) through heat exchangers (not shown). The hydrogen gas itself may also be used as a thermal working fluid.
Hydrogen communication between hydrides A.sub.h and B.sub.h typically require additional piping and valves between the two separate tanks. It would be an advantage to eliminate the thermal mass and excess volume necessitated by such hydrogen pipes and valves.
One characteristic of hydrides that must be dealt with is the change in volume of the hydride as it absorbs and desorbs hydrogen. The volume of the hydride typically changes by about 25% as it goes through a complete absorption/desorption cycle. Metal hydrides also typically are extremely friable and become broken into small particles, of a powder-like form due to repeated hydriding cycles. The powdery hydride material is typically held in a strong pressurized tank, contained in a receptacle termed the hydride bed. If the hydride is constrained in a fixed volume container, the expansion of the hydride as it absorbs hydrogen can exert great stresses on the hydride receptacle.
The hydrogen gas being absorbed and desorbed from the hydride material is a good conductor of heat, tending to keep the particulate hydride material and hydrogen gas nearly in thermal equilibrium as they interact. Thermal losses in a heat pump are due in part to the transfer of heat between the hydrogen gas and the associated pressure containers, pumps, piping and valves, including any heat exchanger fluid and materials.
Discussion
Thermodynamic issues effecting efficiency
The thermodynamic processes involved in the hydriding are controlled mainly by physics. The designer can only select what appear to be appropriate hydride metals for the task at hand with prior art hydride heat pump configurations. There are however, thermodynamic considerations that effect the efficiency and therefore operational cost of a hydride heat pump.
The first consideration deals with the reduction of thermal mass. All of the physical components of the heat pump (hydride metal, tanks, piping, working fluid exchange pumps and working fluid etc) have some mass and each component mass has an individual specific heat-that amount of heat (in Joules) required to raise the mass one degree centigrade. That heat is stored in the mass or given up as the temperature of the mass is cycled and usually does no useful work. It is therefore waste heat. Waste heat can be reduced by using smaller masses of materials and using materials with lower specific heat values.
Previous art heat pumps typically use two separate pressure containers (tanks) to isolate the two hydrides. This results in a large amount of extra mass and subsequent extra waste heat per cycle. It would be an advantage to reduce the amount of mass of the pressure containing vessels, piping, and pumps to reduce wasted heat.
Reducing the amount of hydride metal needed is another method of reducing the thermal mass and waste heat. It would be an advantage to cycle smaller amounts of hydride metal more frequently in order to pump a given amount of heat in a given time.
The second thermodynamic consideration is the thermal resistance between the each particle of hydride and the external thermal sink or source. Heat transfer is controlled by the temperature difference between two objects (hydride metal and source/sink) and by the thermal resistance between them. The lower the resistance, the higher the heat flow for a given temperature difference. The temperatures are usually dictated by the environment, hydride operation etc. The thermal resistance can be controlled by design. Thermal resistance is most simply defined as ##EQU1##
The thermal conductivity of the material is given in terms of heat flow per unit area (through which the heat flows) per degree of temperature per unit time. It is actually a measure of rate of heat flow through the material. Low thermal resistance results from high thermal conductivity, large area over which the heat can flow and minimal thickness through which it has to travel. The significance of low thermal resistance is that a greater amount of heat per unit time can flow, speeding up the transfer of heat into and out of the hydride material (hydride bed).
The third thermodynamic consideration design option deals with the reduction of thermal leakage, i.e. excess or unwanted heat flow. Heat that flows back and forth directly between the hydride beds at different temperatures without being transferred through the hydriding reaction is wasted and constitutes heat loss from the system. The leakage reduces the amount of useful heat "pumped" by the hydrides exchanging hydrogen. It is an advantage to increase the thermal isolation of the different hydride beds (first and second hydrides) which reduces or eliminates this source of leakage.
It would be advantageous to provide a hydride bed system which results in pumping less heat per cycle (heat loss) from the heat source and pumping the useful heat much faster (lower thermal resistance) increasing the cooling (heating) rate for a given size heat pump.